Jensen Landscape Services

Managing irrigation for slopes requires frequent monitoring and adjusting of schedules -- especially if there are mixed plantings on one irrigation zone, (such as shrubs with a trailing groundcover).

The principles for slope irrigation are basically the same as for any irrigation system except for the following key issues, which require special attention:

• Utilize sprinkler heads with a low precipitation rate (rotors and stream rotors for example).

Regardless of soil structure, the slope will encourage runoff. The precipitation rate (rate at which water is applied) should not exceed the rate of infiltration (rate at which water moves into the soil profile).

• Great care must be given to mounting heads for the most even application of water.

Heads that are perpendicular (right angle) to the slope tend to shoot water straight up into the air, for very poor uphill coverage. Sprinklers mounted vertically tend to shoot water into the slope on the uphill side, and broadcast water over the intended crop on the downhill side. Heads set at a mid-point angle between vertical and a right angle to the slope seem to offer the most effective compromise. These rules-of-thumb should be fine tuned to your particular site conditions!

• For irrigating trees and shrubs, consideration should be given to drip irrigation or bubblers.

Both of these approaches offer distinct benefits and some constraints. Typically they require diligent monitoring for purposes of scheduling and repairs. The benefit is that the water tends to be more accurately placed into the rootzone of the intended plant material. However, these types of hardware can be more vulnerable to vandalism or damage, and their effectiveness can be readily undermined by an unskilled (though well-intended) repair.

• The irrigation zone layout should consider the appropriate grouping of plant type and micro-climate, but also irrigate the top and toe (bottom) of the slope separately.

This enables a more effective management of the system because typically the toe of the slope does not require as long an irrigation as the top does. In addition, there are two important design parameters to adhere to:

- The first is providing appropriate pipe sizing to avoid more than 10% variation in operating pressure between heads within the station zone.

- The second is sizing pipe so that the velocity of water does not exceed the industry standard of five feet per second (fps). A practical solution to this problem is running the piping across the slopes (whenever possible), rather than up and down the slopes.

All of this assumes that the appropriate calculations have been made to ensure adequate pressure and volume from the source to the last and highest head with in the system!

• An important component of any irrigation system (particularly on slopes) is a spring check valve (or valves) in the laterals, or check valves in the base of the sprinkler heads. This device will mitigate or prevent the drainage of water (after the valve has closed) through the lowest head in the system.

• Utilize a controller that has multiple programs and cycle starts.

In order to fill the rootzone of the plants, one irrigation application (ending before the point of runoff) typically will not be adequate, especially during the active growing season. This means that several applications (cycles) may need to be made to comprise one irrigation event. Ideally, different hydrozones (plant groupings of similar water requirements) and micro climates (site specific variations in climate conditions) should be irrigated by separate irrigation zones and managed by separate irrigation schedules.

John Ossa is certified by the Irrigation Association for Commercial Irrigation Design and Irrigation Auditor. In addition, he holds a California State Landscape Contractor license, and has been certified by the AWWA to test backflow devices. Industry affiliations include Chairman of the Irrigation Association Water Management Committee, and he is a member of the Irrigation Association Board of Directors.